A plurality of patents has disclosed gain tuning methods of PID controllers. These patents may be categorized into two types of methods; one is a gain tuning method of PID controllers using hardware equipments and the other using software algorithms.
Among these patents, the second type of methods, i.e., gain tuning methods using software algorithms, to which the present invention pertains, can be exemplified as follows: U.S. Pat. No. 6,081,751 titled “System and method for closed-loop autotuning of PID controllers”, U.S. Pat. No. 5,971,579 titled “Unit and method for determining gains of a PID controller using a genetic algorithm”, U.S. Pat. No. 5,742,503 titled “Use of saturation relay feedback in PID controller tuning”, U.S. Pat. No. 5,331,541 titled “PID control unit”, U.S. Pat. No. 5,229,699 titled “Method and an apparatus for PID controller tuning”, U.S. Pat. No. 5,057,993 titled “Method and system for acquiring parameters in process control”, U.S. Pat. No. 4,754,391 titled “Method of determining PID parameters and an autotuning controller using the method”, and U.S. Pat. No. 4,466,054 titled “Improved proportional integral-derivative control apparatus”.
Based on online tuning, the above-mentioned patents are directed to methods for autotuning of PID controllers using algorithms which measure set point values and process variables to suggest suitable gain values.
U.S. Pat. No. 6,081,751, as shown in FIG. 1A, discloses a method for calculating new PID controller parameters either directly through the formulae associated with the Ziegler-Nichols reaction curve method or through the intermediate step of calculating an ultimate period and frequency from the time constant and dead time which are calculated from the period and amplitude of oscillation generated by a relay.
U.S. Pat. No. 5,971,579 is directed to a method for determining gains of a PID controller utilizing a genetic algorithm unit shown in FIG. 1B.
U.S. Pat. No. 5,742,503 provides a method for autotuning parameters of a PID controller, wherein parameters of a transfer function are computed through two steps and precise parameters of the PID controller are determined from the computed parameters of the transfer function.
U.S. Pat. No. 5,331,541 discloses a PID control device which identifies the rise characteristics of a controlled system by a step response method on changing a reference, moves to PID control when idle time and slope successively obtained on the rise reach a predetermined value, and computes PID control parameters based on the idle time and slope obtained up to that point.
U.S. Pat. No. 5,229,699 suggests a method for tuning PID controllers, in which a proportional control gain is increased until a desired oscillation is obtained, an amplitude and period are measured from the oscillation, an ultimate gain and an ultimate period is calculated in accordance with the amplitude and period, and the parameters of the PID controller are set in dependent upon the ultimate gain and period.
U.S. Pat. No. 5,057,993 introduces a method for acquiring parameters in the process control. According to this method, a manipulated variable to which an identification signal from an identification signal generator is added is inputted to a process to produce a controlled variable output which is then sampled to obtain a dead time and a maximum gradient, and the initial values of PID control parameters are calculated on the basis of the dead time and the maximum gradient. The initial values of the PID control parameters and the like are set in the adaptation section. In the adaptation section, a pulse transfer function of the process is acquired, and PID control parameters are calculated from the acquired pulse transfer function by using a partial matching method in a frequency region.
U.S. Pat. No. 4,754,391 discloses a method of determining the PID parameters for the PID controllers by monitoring a limit cycle generated in a controlled process to obtain characteristics of the process and determining optimum PID parameters to be used for succeeding process control on the basis of the results of the limit cycle monitoring.
U.S. Pat. No. 4,466,054 suggests a process control apparatus which comprises a nonlinear controller connected in parallel with a PID controller, a circuit for identifying a dynamic characteristic of a process and a circuit for determining gains of the PID controller according to the dynamic characteristic, which is illustrated in FIG. 1C.
All of the prior arts described above suggest methods for autotuning a PID controller to acquire a desired response during an online control process and realization of the methods in a hardware system.
Such conventional methods, however, require additional systems, thereby making the structure of PID controllers to be complicated. Further, these methods suffer from the disadvantage of requiring a lot of effort for calculating necessary intermediate values.
In addition, the object systems to which these conventional arts can be applied are limited to linear systems.